It is frequently necessary to visually inspect electronic parts for defects and quality. In particular, many types of electrical components used in various military projects must undergo various types of inspection and/or test. One inspection which components must undergo is that required by Military Specification 202F. That specification requires that leads be pretinned and inspected. Quality control guidelines dictate that thirteen leads out of each component lot be inspected for 95% solder coverage.
The prior art inspection systems are in large part manual and highly labor intensive. Human operators visually inspect the leads with a 10.times. microscope. The operator positions the lead under the microscope, and estimates the amount of solder coverage. The estimate requires the operator to determine the area of the lead covered by defects. This visual method has serious drawbacks caused by operator eye fatigue and difficulty in estimating the area of the lead covered by defects. Furthermore, the method averages between 10 and 30 seconds depending on the lead size.
The prior art inspection methods utilized incandescent or flourescent lights positioned above the components and around the eyepiece of the microscope, or fiber optic light rings positioned around the objective lens of the microscope above the lead. All of these lighting techniques produced lead images with numerous reflective glare spots and dark spots, regardless of the actual solder coating on the leads. The appearance of the uncoated or defective areas of the lead changed depending upon how the lead was positioned relative to the light source and the lens.
The change in appearance contributed to the problems with visual inspection systems, particularly with respect to consistency of results.